PURPOSE: To prepare a propylene polymer or copolymer having high melting tension, self-tack and improved transparency, by using a polymerization catalyst system comprising a titanium compound supported on a magnesium compound, an alumium compound and an electrode donor.

CONSTITUTION: A (co)polymer of propylene, etc. is prepared by using a polymerization catalyst system comprising (A) a solid component of (a) a magnesium compound supporting (b) a titanium compound (e.g. titanium tetrachloride, etc.), (B) an alumium compound R1pAlX3-p (R1 is 1W20C hydrocarbon; X is halogen; 0<p≤3) and (C) an electron donor (e.g. hexamethylphosphoric triamide, etc.). The component (a) is preferably the reaction product of an organomagnesium compound with a compound R2nAlX3-n (R2 is 1W20C hydrocarbon; 0≤n<3) and/or a compound R3mSiX4-m (R3 is 1W20C hydrocarbon; 0≤m<4).